The present invention relates to the general field of servo-valve actuator systems for an aircraft turbojet. More particularly, the invention relates to a method of monitoring failures on torque motors in such actuator systems.
A turbojet typically includes servo-valve actuator systems for controlling the various pieces of equipment and members of the engine or its environment that have dimensions, shapes, positions, and/or speeds that need to be modified as a function of detected events or of operating parameters of the engine. By way of example, these may be variable pitch stator vanes for compressor stator stages, compressor bleed valves, or indeed fuel metering valves.
Typically, these actuator systems are controlled by the electronic regulator module of the electronic engine control (EEC) so as to adapt the controlled members to the flight scenario. In known manner, servo-control loops serve to monitor and control actuator systems in which degradation can lead in particular to members of the turbojet taking up positions that do not comply with commands under steady conditions, or to those members responding only slowly to commands under transient conditions.
Such degradations constitute the early stages of failures, since in general they can be compensated initially by the servo-control loops or merely left without being corrected, having no consequence other than reconfiguration (e.g. changing an active control channel).
Nevertheless, after a certain length of time, when such degradations persist and become worse, they can no longer be compensated due to limitations of the control equipment. Thus, degradations can have the consequence of making the turbojet inoperable or inefficient. That can lead to a failure message being issued. Such deteriorations are thus detected too late since they are not detected until the actuator system has failed. A particular type of degradation concerns the torque motors of servo-valves, such as those that can be used for controlling a fuel metering valve. These are actuators that are current controlled by the engine regulator module or computer, with the current supplied to the torque motor controlling the output flow rate from the actuator.
Faults are detected in such actuators on the basis of comparing the control current requested by the computer with the current returned to the computer. Unfortunately, in the event of a differential short circuit (i.e. across the terminals of the torque motor), the current remains unchanged even though there is a failure.
There therefore exists a need to have an effective method of monitoring an actuator system of a turbojet that might be the subject of a failure such as a differential short circuit, in particular in order to be able to issue a maintenance message concerning the actuator system before the turbojet is made inoperable or inefficient.
A similar need exists in the other applications for a servo-valve actuator system.